Series-wound machine with additional excitation in bridge connection



l 621 429 March 15, 1927. W. PETERSEN SERIES WOUND MACHINE WITH ADDITIONAL EXGITATION IN BRIDGE CONNECTION Filed 001;. 27, 1925 Patented Mar. 15 1927.

UNITED STATES WALDEMAR PETERSEN, OF DARMSTADT, GERMANY.

SERIES-WOUND MACHINE WITH ADDITIONAL EXCI'IATION IN BRIDGE CONNECTION.

Application filed October 27, 1925, Serial No. 65,138, and in Germany October 28, 1924.

' This invention relates to series wound generators and has for its object to provide a generator of this type which will produce a constant voltage at all speeds.

Since the building of the first direct current machine it has been the endeavor in the electrical art to produce a direct current dynamo which would give a uniform voltage in spite of variation in speed of revolution, load, and temperature, in order to meet the demands of lighting systems, particularly those of the incandescentbulb type. The need of means for maintaining a constant voltage occurs particularly in train lighting systems, in which the dynamos are driven by the car axles, the speed of which necessarily varies, and in systems in which dynamos driven by ungoverned water motors are used.

Heretofore, to overcome the changes in voltage in ordinary shunt wound dynamos due to changes in speed, shunt voltage regulators have been used, but these all had the technical disadvantage of requiring moving parts and contact devices. By the present invention constant voltage is secured by means ofva boosting exciting winding which forms an arm of a Wheatstone bridge connected across the main circuit.

The invention will be described in connection with the accompanying drawing in which one form of the invention is diagrammatically represented.

The reference letter or represents the armature of a series wound generator of which 71. is the main field winding, which is so constructed as to produce a predetermined voltage at the lowest speed and normal current. The voltage can be adjusted by means of resistances in the shunt circuit 6. A boosting coil 5, which forms an arm of a Wheatstone bridge, is interposed in the main winding h. The Wheatstone bridge consists of arms 1 and 3, which are made as resistances dependent on voltage, that is, whose resistance varies with the applied voltage, and the arms 2 and 4, whose resistance is likewise dependent on voltage, but to a lesser extent, or in a reversed sense. The connections of the arms 1 and 4, and 2 and 3 lie in the armature circuit. The boosting winding 5 is connected to the connections of the arms 1 and 2, and 3- and 4. Every change in the potential difference between the bridge points 14 and 2-3 immediately creates a change in the potential difference between the bridge points 1 -2 and 3-4, and thereby causes a variation of the current in the auxiliary exciting winding 5. The current in this winding flows according to the conditions either in the same direction as the current in the main winding h, or in opposed direction thereto, andthe additional excitation therefore needs to be only a very small part of the total excitation.

The bridge can be so adjusted to the fundamental speed and full-load terminal voltage that it produces a positive or an opposed excitation, or no excitation in the boosting coil 5. By this simple means variation of the characteristic can be obtained. This is shown particularly clearly in an important use of the dynamo, namel as a generator of constant voltage driven y an ungoverned water motor. As is well known the idling speed of a water motor (water-wheel, Pelton wheel, or turbine) is about 1.8 times the full load speed. If it could be assumed that the speed increases and decreases in inverse proportion to the load, and that -the voltage characteristic of the dynamo is made a straight line by a corresponding choice of saturations, by so choosing the main current excitation that with a small load, and 35 correspondingly higher speed, the same normal voltage is maintained as with the lower full load speed, with correspondingly higher load and exciting currents, the generator would produce a constant voltage over the 90.

whole range of speed. But as a matter'of fact the curve of the dependence between load and speed in a water motor is a para-' bola, for as full load is approached the speed decreases faster than the load in- ,06

creases, while in the vicinity of racing speed the speed increases slower than the load decreases. On account of this characteristic of water motors the above mentioned assumption is not correct and a series wound 1 motor will not produce a constant voltage at various speeds Without something further.

If in accordance with the invention the main field winding is supplemented by an auxiliary winding which is made very, sensitive to changes in voltage between its terminals b the Wheatstone bridge, the differences etween the water motor characteristic and the dynamo characteristic are equalized, and a practically uniform voltage of no the generator is obtained.

The same observations apply to generators speed tion that they will adjust themselves to their I driven by any'source of power which does not ive a load. In these cases the total excitation must be made to correspond with the speed inverse .ratio by corresponding adjustment of the main current excitation by the load current and of the boosting excitation by the boosting coil current, which is dependent upon the relationships of the Wheatstone bridge. .In all' these" eases the dependence of voltage upon the temperature of theigen eratorwindings' 'is very slight," due to the far greatereffect of themain current excitatiolL r It 1 is of the greatest importance for the rapid adjustment of the uniform terminal vo tag'e under variations in speed,yand for the maintenance of such voltage, that the resistances 1, 2, 3,'and 4 be of such construcstationary temperature after a change in current or voltage in the shortest possible'time, I

For *examplefif resi tances "2; and if were made in whole or in-part as exciting windings, variations from the desired voltage of as much as 20% would be possible, since as is' well known, the resistance of exciting windingsvariesonlyslowly with Variations in load. Theuse of resistances witli'out any thou ht as to thelengthof time they require to a just themselves to the necessary temperature'willlead to no useful technical result. According to- 'the present invention therefore, resistances are used whosevalu'e is adjusted practically instantaneo ly w variations in voltage. a r i Ifthe resistances of satisfy-these requirements with very slig the wheatstone bridge t changes in voltage in the armature a). large'current's" will flow in the bridge arm or boosting coil 5. For example resistances 1 and 3may= bev positively dependent upon voltage" and increase with increasing voltage (e.g. iron'resistances, or metal filament slf nd the resistances 2 and 4 maybe constant speed of rotation, or a varying in exact dependence upon the constant,-or negatively dependent upon volt age (e.-g. carbon-likeresistances. or carbon filament lamps).

* The operation of the Wheatstone brid e can be'furtherinflue'nced and increased y an additional .heating of the resistances 1 I the resistances ands, or 2' aud t, or even'all by meansoi the useful current of the machine or a part of the same. In thedrawing coils. 7 and 8 connected in series in the main circuit andarrangedin heat exchange relation with the'resistances 1 and 3. The heating of the iron resistances bythe useful current ofiers among other things the possi-' bility of reducing the current requirement of the bridge for the same voltage characteristic.

Having thus described my invention, 1 claim: i v

1. In a series wound machine, a Wheatstone bridge .the adjacent resistances of which are difi'erentially variable under a change in voltage, connections between two opposite .connectionsof said resistances and ring are shown by way "of example the heat the terminals of the machine,and a boosting chine. s v

,4. ArSBI'lGS wound machine according to claim 1 in combination with electrically ing theresistances of theWheatst'one bridge by means of: the useful current of the maoperable-heating means for the resistances of th' 'Wheatstone bridge and means for supplying current'fromthe system for operating said heating'means.

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